High-strength steel sheets have been used for vehicle bodies for the purpose of reducing the weight of the vehicle. In recent years, these high-strength steel sheets have been required to have both reduced thickness and high dent resistance. To respond to these requirements, bake-hardenable cold-rolled steel sheets have been used.
The bake-hardenable cold-rolled steel sheets have yield strength close to that of a soft steel sheet, and hence, exhibit excellent formability at the time of press forming. Further, a coating and baking process is applied after the press forming to enhance the yield strength. More specifically, the bake-hardenable cold-rolled steel sheets have both high formability and high strength.
The baked hardening utilizes a sort of strain aging in which dislocation occurring during deformation is fixed by carbon in solid solution or nitrogen in solid solution, which are interstitial elements solid solved in steel. The amount of baked hardening (BH amount) increases with the increase in the the amounts of carbon in solid solution and the nitrogen in solid solution. However, if the solid-solution element excessively increases, the formability deteriorates due to the cold aging. Thus, it is important to appropriately control the solid-solution elements.
Conventionally, for the bake-hardenable cold-rolled steel sheet, attention has not been paid to the change in the r value (Lankford value) serving as an index for deep-drawability or the |Δr| value indicating the planar anisotropy of the deep-drawability depending on the Mn and P added for enhancing the strength of the steel, or on the Mo added for increasing the cold aging resistance.
Conventionally, various bake-hardenable cold-rolled steel sheets have been proposed. For example, Patent Document 1 and Patent Document 2 describe a bake-hardenable high-strength cold-rolled steel sheet and a method of manufacturing the bake-hardenable high-strength cold-rolled steel sheet, in which solid solution strengthening of an ultralow carbon steel having Nb added therein is achieved by adding Mn and P; the bake hardenability is imparted by adjusting the amount of C in solid solution while taking the balance between the amount of C and the amount of Nb into consideration; and the cold aging resistance is imparted by adding Mo. However, the above techniques are made on the basis of the idea of utilizing the grain boundary carbon to obtain the bake hardenability by making the microstructure finer, and hence, AlN dispersion is essential. This inhibits the growth of the grain during annealing as well as the recrystallization. Further, in the first place, the amount of Al added is large, and hence, the surface defects caused by oxide are likely to occur. Yet further, these documents do not discuss the deep-drawability such as the r value and the planar anisotropy of the deep-drawability.
Patent Document 3 relates to a bake-hardenable high-strength cold-rolled steel sheet used for automobile outer panels and having cold aging resistance and a method of manufacturing the bake-hardenable high-strength cold-rolled steel sheet, in which a cold rolling reduction ratio is defined with a function of the amount of C added to reduce the planar anisotropy. However, rather than the ultralow carbon steel, Patent Document 3 relates to a steel sheet having a composite microstructure such as DP steel formed by ferrite and low-temperature transformation phase, and seems to relate to a steel having a significantly high strength. Further, the reason for adding Mo as well as Cr and V is to enhance the hardenability of austenite so as to obtain the low-temperature transformation phase. This document does not disclose the r value itself, and the deep-drawability is unclear.